Re: [PATCH] setup_per_cpu_areas in 2.5.8pre3

[Andries.Brouwer@cwi.nl]

    > Now that I am writing anyway, one of the changes I needed
    > to compile 2.5.8pre3 is the following.

    Yeah, better patch below

Good.

    > Of course the real fix is to remove the #ifdef's,
    > maybe using a weak symbol instead, or some other construction
    > that defines an empty default that can be replaced by an actual
    > routine.

    Not unless you make it as readable as the current code.  Having magic
    appearing functions sounds cool, but beware that the cure might be
    worse than the disease.

But maybe I do not think that the current code is very readable.
Probably because just before fixing this compilation problem I had
to fix a different one where atomic_dec_and_lock was undefined,
and one finds a forest of #ifdef's in spinlock.h.

#ifdef's are evil. You have one, and there are two possible sources.
Easy and readable. You have two, and there are four. Already a small
effort to check that indeed all four combinations are OK. That was
what went wrong in the setup_per_cpu_areas case. You have three and
it is almost certain that someone forgets to check all possible
eight cases.

#ifdef's hide source from the compiler, so that when stuff compiles
for the developer it need not compile for the next person who comes
along. We have a kernel compilation project because of that.

So, if we can replace a certain type of #ifdef use by a different
formal construction, where all source is seen by the compiler,
that might well be progress.

Andries